1. Field of the Invention
The present invention relates to a focus detection apparatus for, e.g., a camera.
2. Related Background Art
As a conventional focus detection system in a single-lens reflex camera, a phase difference detection system is known. This system will be explained below with reference to FIG. 25.
A light beam incident through an area 21a of a phototaking lens 21 is focused on a photoelectric transducer array 41a via a field mask 31, a field lens 32, an aperture stop portion 33a, and a re-focusing lens 34. Similarly, a light beam incident through an area 21b of the phototaking lens 21 is focused on a photoelectric transducer array 41b via the field mask 31, the field lens 32, an aperture stop portion 33b, and a re-focusing lens 35.
In a so-called near-focus state wherein the phototaking lens 21 forms a sharp image of an object before a prospective focal plane, a pair of object images focused on the photoelectric transducer arrays 41a and 41b are separated away from each other. Contrary to this, in a so-called far-focus state wherein the phototaking lens 21 forms a sharp image of an object after the prospective focal plane, the pair of object images approach each other. In a so-called in-focus state wherein the phototaking lens forms a sharp image of an object on the prospective focal plane, the object images on the photoelectric transducer arrays 41a and 41b coincide with each other. Therefore, the pair of object images are photoelectrically converted into electrical signals by the photoelectric transducer arrays 41a and 41b, and the converted signals are subjected to arithmetic processing by a microcomputer (not shown) so as to obtain the relative position of the pair of object images. Thus, a focusing state, in this case, a shift amount from the in-focus state, and a shift direction (to be referred to as a defocus amount DF hereinafter), can be obtained.
In a conventional apparatus of this type, as shown in FIG. 26, areas (r.sub.1) to (r.sub.3) of about .+-.2 to .+-.3 mm having the position (r.sub.2) intersecting an optical axis LX of the phototaking lens as the center on a focus detection plane 50 are used as focus detection areas, and images within this range are focused on the photoelectric transducer arrays 41a and 41b on an IC board 41 using the pair of re-focusing lenses 34 and 35. A relative image shift amount is detected based on image outputs from the two photoelectric transducer arrays 41a and 41b, thus performing focus detection.
A focus detection apparatus of a single-lens reflex camera must prevent vignetting (eclipse) in a focus detection optical system even for an exchangeable lens having a full-open f-number of about F5.6. In most cases, the exit pupil position of an F5.6 phototaking lens is present somewhere in a range of about 50 mm to 200 mm indicated by hatching. Thus, a spread .alpha. of a detection light beam must be set to be about F7 so as to prevent vignetting in an F5.6 lens which has an exit pupil position within this range.
When the apertures of the re-focusing lenses 34 and 35 are projected at the exit pupil position of about 100 mm (a range L1 in FIG. 26) by the field lens 32, light beam components (broken lines) passing through points (r.sub.1) and (r.sub.3) having an image height of 2 to 3 mm from the optical axis can be subjected to focus detection without causing vignetting for an F5.6 lens having an exit pupil position of 50 mm to 200 mm. With this structure, in a conventional focus detection apparatus, focus detection free from vignetting can be performed within a range of an image height of about 3 mm.
In some cases, focus detection must be performed using areas (r.sub.4), (r.sub.5), and (r.sub.6) separated from the center of the optical axis by 3 to 4 mm or more as focus detection areas. Thus, re-focusing lenses 61a and 61b, and an IC board 71 are arranged, as shown in FIG. 26, so as to define the positions (r.sub.4), (r.sub.5), and (r.sub.6) having the point (r.sub.5) of an image height of 7 mm as the center as focus detection areas. In this case, this focus detection optical system is constituted, so that the apertures of the re-focusing lenses 61a and 61b form conjugate images at the exit pupil position of about 100 mm by the field lens 32. As can be apparent from FIG. 26, when an F5.6 phototaking lens is used for the focus detection areas (r.sub.4), (r.sub.5), and (r.sub.6), vignetting can be prevented only when the exit pupil position of the phototaking lens falls within the range L1 of about 100 mm. Therefore, an exchangeable lens which can be used is limited, thus posing many problems in practical use.